This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Typical mining shovels or excavators use a dipper assembly to scoop earth or other material from horizontal or vertical faces. These conventional mining shovels typically have a fixed boom, and the dipper assembly is mounted on the boom via a crowd mechanism. The fixed boom extends upwardly and outwardly from the frame. The dipper assembly is typically forced into a generally vertical face (i.e. a bank, etc.) by a crowding force, digging into the bank and filling the dipper assembly with material. Once the dipper is filled with material, the dipper is moved to a deposit point to unload the material.
As the dipper assembly digs into the bank, the shovel operators are able to continue to apply a crowding force, perhaps excessively, forcing the dipper assembly into the bank. This excessive crowding force can cause damage to the dipper assembly or to other components of the mining shovel. The excessive crowding force can also cause a “boom jack” condition, where the boom pivots and “jacks” away from the ground as a result of the excessive force. Also, following loading of the dipper, operators often attempt to swing the dipper assembly before the assembly is sufficiently free of the bank. This swing force can cause the mining shovel to twist, creating a torsion force or compressive loading force on the mining shovel. This excessive force can also damage the mining shovel or its components.
Conventional mining shovels may include a crowd force control to prevent excessive crowd force. An example of such a conventional crowd force control can be found in U.S. Pat. No. 7,519,462 issued Apr. 14, 2009, for “Crowd Force Control in Electrically Propelled Machine.” This patent discloses a method for limiting the output torque of an electric propulsion motor based upon the occurrence of a crowd force condition of the machine. This conventional method limits crowd force based on a crowd force threshold, however, and is susceptible to damage by an applied crowd force below the specified threshold.